Prism coupling is one of the most common methods to test integrated optical circuits and structures. It is used in connection with methods to determine the effective refractive indices and thicknesses of the waveguides, by means of the structure of the propagating modes of light within the waveguide. This technique is described in the publication by Tien, P. K., Ulrich, R., and Martin, R. J.: Modes of propagating light waves in thin deposited semiconductor films, Applied Physics Letters 14 (1969) 9, pp. 291-294. Prism coupling is also required in measuring distributions of propagating modes and attenuation in the structures.
The device by which the testing is to be performed comprises a source of light, such as a laser, a prism having an unbeveled corner to which the incoming ray of light is directed at a desired angle, as well as measuring devices such as a light detector and a CCD-camera situated perpendicularly to the waveguide. The light must be directed to the prism at a characteristic angle pertaining to one of the propagating modes in the waveguide. This requires that the source of light be turned with respect to the prism, e. g. by means of a goniometer. Since the source of light is often relatively large, its precise positioning places great demands on the fine mechanism of the device and at the least makes the whole device large and inflexible to varied geometries.